1. Field of the Invention
The present invention relates to a semiconductor device preferable for a ferroelectric memory, and a manufacturing method thereof.
2. Description of the Related Art
In recent years, with development of the digital technology, there has been a growing trend to process or store a large capacity of data at a high speed. Therefore, high integration density and high performance of the semiconductor devices used for electronic equipment are required.
Thus, concerning a semiconductor device, in order to realize, for example, high integration density of a DRAM, the technique using a ferroelectric material or a high dielectric constant material in place of the conventional silicon oxide or silicon nitride as a capacity insulating film of a capacitor element composing a DRAM starts to be researched and developed on a wide scale.
In order to realize a nonvolatile RAM capable of write operation and read operation at a lower voltage and a higher speed, the technique of using a ferroelectric film having spontaneous polarization characteristics as a capacity insulating film has been increasingly researched and developed. Such a semiconductor memory device is called a ferroelectric memory (FeRAM).
A ferroelectric memory stores information by using a hysteresis characteristic of a ferroelectric. The ferroelectric memory includes a ferroelectric capacitor, and the ferroelectric capacitor is constructed by sandwiching a ferroelectric film as a capacitor dielectric film between a pair of electrodes. The ferroelectric film causes polarization in accordance with an applied voltage between the electrodes, and has spontaneous polarization even after the applied voltage is removed. If the polarity of the applied voltage is inversed, the polarity of the spontaneous polarization is inversed. Accordingly, if the spontaneous polarization is detected, information can be read. The ferroelectric memory operates at a lower voltage as compared with a flash memory, and is capable of writing at a high speed with power-saving.
Now, use of a logic-mounted chip (SoC: System on Chip) including a ferroelectric memory for an IC card or the like is studied.
As a ferroelectric film, a film of a PZT material, a film of a Bi-layer structure compound, and the like are used. As the PZT material, lead zirconate titanate (PZT) itself, a PZT film doped with La, Ca, Sr and/or Si, and the like are cited. As the Bi-layer structure compound, SrBi2Ta2O9 (SBT, Y1), and SrBi2(Ta,Nb)2O9 (SBTN, YZ), and the like are cited. The ferroelectric film is formed in an amorphous state or a microcrystal state on the bottom electrode film by a sol-gel method, a sputtering method or the like, and thereafter, is crystallized by heat treatment. It is sometimes formed in a crystallized state on the bottom electrode by an MOCVD (Metal Organic Chemical Vapor Deposition) method.
In recent years, the ferroelectric memory has been applied to portable information processing equipment such as a cellular phone. Therefore, operation at a low voltage is required of a ferroelectric memory. Thus, a large switching charge amount is required. However, a ferroelectric film has the property that oxygen deficiency easily occurs due to processing under a non-oxidizing atmosphere and the characteristics of a switching charge amount (inversion charge amount), a leak current value and the like easily degrade. On the occasion of forming a ferroelectric capacitor, oxygen deficiency, damages and the like easily occur to the ferroelectric film. Therefore, in order to recover them, heat treatment (recovery annealing) under an oxygen atmosphere is performed a plurality of times. Thus, as the material of the top electrode, a metal such as Pt, which is hardly oxidized under an oxygen atmosphere, and a conductive oxide such as IrOX and RuOX is used.
On the other hand, microfabrication is also required of a ferroelectric memory, and a multilayer wiring structure is being adopted. However, in the case of adopting the multilayer wiring structure, the processing under a reducing atmosphere or a non-oxidizing atmosphere is required. For example, when the multilayer wiring structure is adopted, an interlayer insulating film is sometimes formed under a reducing atmosphere including hydrogen. Accordingly, even when recovery annealing is performed in forming a ferroelectric capacitor, the characteristics of the ferroelectric film sometimes degrade thereafter. This is especially conspicuous when Pt or Ir is used as the material of the top electrode. This is because Pt has catalysis, hydrogen which diffuses into the top electrode including Pt is activated, and by this effect, the ferroelectric film is reduced. If the ferroelectric is reduced, the characteristics of the ferroelectric capacitor significantly degrade. Such degradation of the characteristics becomes conspicuous as the ferroelectric capacitor is microfabricated, and the capacitor insulating film in the ferroelectric capacitor is microfabricated.
Thus, adoption of a conductive oxide is conceivable, but another problem occurs though reduction can be suppressed in the conductive oxide conventionally used. In a stack type ferroelectric capacitor, a contact plug is formed on the top electrode, and its base film (a Ti film, a TiN film and/or the like) is oxidized by the influence of oxygen included in the conductive oxide. Occurrence of such oxidization increases contact resistance between the top electrode and the contact plug.
As described above, in the related art, the problem of reduction of the ferroelectric film occurs in the case of using Pt or the like for the top electrode, and the problem of rise of the contact resistance occurs in the case of using a conductive oxide. These problems are not solved in the arts disclosed in Japanese Patent Application Laid-open No. 11-195768, Japanese Patent No. 3661850, Japanese Patent Application Laid-open No. 2000-91539, Japanese Patent Application Laid-open No. 2000-173999, Japanese Patent No. 3299909 and Japanese Patent Application Laid-open No. 2005-93605.